Optical pulses having a duration of a few femtoseconds or less may include only a few optical cycles at a fundamental carrier frequency of the pulse within an envelope of the pulse. A pulse-envelope typically has a Gaussian or Sech-squared form. The peak power within the envelope will depend on the phase of the carrier cycles relative to the envelope. This is referred to by practitioners of the art as the carrier envelope phase (CEP). FIG. 1A is a graph schematically illustrating a condition where the carrier is retarded in phase by an amount φCE with respect to the pulse envelope. The highest peak power will occur when a peak of one of the carrier cycles is exactly in phase φCE=0.0) with the peak of the envelope. This is schematically illustrated in FIG. 1B. The less the number of cycles within the envelope, i.e., the shorter the pulse, the greater is this phase dependence of peak power in the pulse.
Techniques for stabilizing the CEP of a laser oscillator have long been known in the art. One such technique involves a closed loop feedback arrangement wherein the CEP is measured and compared with a desired value. Any difference between the measured and actual value is used to vary optical-pump power to a gain medium of the oscillator to drive the measured value back to the desired value. It has been found, however, that if a pulse from a CEP-stabilized oscillator is amplified in a chirped pulse amplification arrangement, wherein the input pulses from the oscillator are temporally stretched from an original duration before amplification and temporally compressed back to about the original pulse duration, the CEP of the amplified pulses will usually not be stable.
Temporal stretching and compression are typically effected using a separated parallel pair of gratings. One means of stabilizing output pulses from a chirped pulse amplification arrangement is disclosed in PCT publication WO 2007149956. Stabilization is effected by a closed loop arrangement in which the CEP is again measured and compared with a desired value. Any difference between the measured and actual value is used to vary the separation of gratings in the pulse stretcher or compressor of the amplifier to drive the measured value back to the desired value. Piezoelectric transducer (PZT) actuators are used to change the grating separation. It has been found practically that a grating mount with three PZTs is required to translate a grating and maintain parallelism. The PZTs must be identical such that each provides the same displacement for an applied electrical potential. As the CEP is relatively insensitive to change in separation this method stretches the limits of displacement that can be provided by PZTs. There is a need for a more sensitive CEP stabilization method for chirped pulse amplifiers that does not require a plurality of matched PZTs.